This invention relates to expansion units made from heavy sheet metal for use in supporting a ceiling or roof, especially a mine roof. In the use of these and similar units, holes are drilled vertically into the mine roof usually in the most stable part of the terrain. A roof bolt is directed through a supporting plate engaged with the roof into the pre-drilled hole. The roof bolt assembly is a threaded rod which is anchored by means of an expansion unit inserted into the pre-drilled hole. The expansion unit contains a threaded wedge or plug for receiving the roof bolt. By threading the bolt into the expansion unit, the latter is caused to expand in the hole and to firmly grip the sides thereof. A compression of the strata in the mine roof results from the tightening of the threaded rod anchored at one end by the expansion device and the plate anchored at the bottom of the rod and at the ceiling proper.
By spacing such expansion units 4, 8 or 12 feet apart and by using a predetermined torque of 150-220 foot pounds in the threading of the bolt, internal pressures of up to 16,000 psi or higher are commonly generated. Because of the magnitude of these forces, commercially successful expansion units must meet the following performance criteria; (1) they must maintain their physical integrity without serious deformation or ripping; (2) they must not rotate in the hole when torque is applied to the threaded bolt so that maximum and efficient expansion can be obtained; (3) they must anchor themselves in the hole sufficiently to avoid being pulled downwardy even for fractions of inches. They must not bleed off (lose friction).
Heretofore most expansion shells have been formed from malleable iron castings due to non-uniform thickness and contour of the components needed to provide a suitable wedging action. Attempts to simulate these castings with shells formed of sheet metal have met with numerous difficulties due to metal failure under the large forces present. This invention is directed to the fulfillment of this need; i.e. a suitable expansion unit made from sheet metal which is equal to or better than the iron casting type and can maintain its physical integrity and holding power in various types of rock strata.
Various means have been attempted in the prior art to accomplish this purpose. For example, U.S. Pat. No. 3,250,170, in the name of Norman H. Siegel, and assigned to the same assignee as the instant application, describes an expansion shell formed from heavy sheet metal and provided with a series of "crater-like extrusions" punched outwardly from the metal. When a wedge is driven downwardly, these extrusions are said to provide anchorage and holding power exceeding the tensile strength of the roof bolt. In actual field use, the Siegel shell has encountered difficulties in certain applications due to failure of the metal where excessive frictional forces have been generated in the harder types of rock strata. Other means have also met with various problems and difficulties. For example, use of expansion units having relatively parallel and smooth sides, in addition to not having the requisite resistance to rotation, will tend to bow in the hole with the bottom of the unit usually collapsing inwardly as a wedge or other means is driven downwardly to expand the unit.